Telephone transmission system.



E. H. COLPITTS TELEPHONE TRANSMISSION S YSTEM. APPLICATION FILED 050.27.1'91'5.

1,200,082. Patented Oct. 3,1916.

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,E. H. COLPITTS.

TELEPHONE TRANSMISSION SYSTEM.

APPLICATION FILED DEC. 27. I915. 1,200.082. Patented Oct. 3,1916.

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To all whom it may concern UNITED STATES PATENT OFFICE.

EDWIN H. COLPTTTS, OF EAST ORANGE, NEW? 'J'ERSE Y, ASSIGNOR TO'WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF YORK.

TTELEPHONE 'raaivsmrssron SYSTEM.

Specification of Letters Patent.

Patented Oct.'3, 1916.

Application flled December 27, 1915. Serial No. 68,845.

'Beit'known that I, EDWIN H. CoLrrr'rs, a subject of the King of Great Britain, residing at East Orange, in the 'county of Essex andState of New Jersey, have invented ,certain new and useful Improvements in Telephone Transmission Systems, of which the following is a full, clear, concise, and exact description. v

This invention relates to telephone transmission systems, and more particularlyto the operation of repeaters on loaded lines. Its object is to increase the amplification at which a telephone repeater may operate with stability under practical loaded line conditions. I This object is accomplished by a suitable correlation of the repeater and line characteristics, giving rise to a certain necessary relation between these characterispedance. Midway between'the line 2 and the artificial line 5'is bridged the winding ,'7 of the repeater coil 8, the secondary of which leads to the actuating winding 10 of tics' which does not obtain in repeaters as hitherto employed. The nature of'this relation will be made apparent from the following discussionof the methods and limits of operation of repeater sets. In this discussion, reference will be made to the drawings, in Which- 3 F igure 1 represents a repeater c rcult employing two repeaters and arranged to give two-way. amplification over a loaded circuit; Figs. 2, 3 and't give curves showing certain loaded line andrepeater character-' istics which must be taken into consideration ifv repeaters are to be operated effectively in connection with loaded lines; and Figs. 5 and 6 show certain details of construction whereby -a repeater may be made suitable for use in connection with loaded lines. I 1

Referring to Fig. 1, there is illustrated a repeater station at which 'two telephone.

lines 2 and 3 terminate. These telephone lines areloaded at regular intervals the well-known manner by means of the load- .ing coils 4, and have therefore what is known as a lumped inductance; 5 and 6 are artificial lines ornetworks designed to bal-' ance the lines 2 and 3 respectively, for ima .telephone repeater of the receiver-microphone type. Telephonic currents therefore, coming'in from the station 11 over the line 2,- will flow through winding 10 of repeaterat station 16 will actuate the winding 17 of repeater B- by 'way of transformer 18, me" by actuating microphone 19 will; send out amplified currents to the station 11 by way of the-repeating-coil- 20. o If the impedance 5 is an exact copy'of the impedance of line 2, no diflerence of potentialwill be set up across the terminals of the winding 7 by currentsflowing' in primary 21 of. the repeating coil 20, inasmuch as the secondary windings 22 are symmetrically dlsposed on either side of the point at which winding 7 is connected. Similarly, if the artificial line 6 has an impedance equal to that of line '3 for all frequencies, and if the windings 15 of transformer 14 are sym metrically disposed with respect to the pri-.

mary of the repeating coil 18, a condition of stability is obtained. It may, however, be shown theoretically, and is also well known in practice,- that if each balancing network is notexactly equivalent at all frequengies to its'corresponding line, a condi-- t1on Wlll be set up in which one repeater feeds .power backinto the other, resulting in a local circulation of power in therepeatersta'tion, When this unbalancing of lines and network becomes sufficiently large,

-the power circulation becomes cumulative,

giving rise to an'unstable operation which is called'singing, and which destroys the usefulness of the device. known that the 'relationbetween the am- It is further plifying' power of the repeaters and the "allowable unbalance between artificial bal anci'ng linesand actual lines is a reciprocal one, so that 1f U, and U are the impedance unbalances of the two lines and their netare related by the equation i a m I works, and if A and A are the amplifying v powers of the two repeaters, these quantities Thus, the maximum amplifying power which maybe used is limited by the close- I ness with which the'lines and networks may balanced, and in' practice this is what 9 limits the gain which may be made with any kind of repeater. If, for any'reason, the line conditions are such that an accurate balance cannot be obtained and singing results, the non-singing condition can be restored by impairing somewhat the amplifying power of the system. A convenient way of doing this is by inserting in the output circuit of the repeaters a shunt 23 by which the amount of energy delivered by repeater A, forfexample, to line 3 is limited. Such a shunt preferably may be in the form of an Ayrton shunt, having a variable shunt member 24 and a variable series member 25, the steps of which are so arranged as to keep the total impedance of the repeater, as viewed from the terminals of the transformer winding 13, constant.

Now, in practice, the impedance of a loaded line changes with frequency inthe manner shown by the full line curve of Fig.

2, which represents the relation between frequency f and line impedance Z. It is found impossible to exactly simulate this impedance by means of a balancing network, although its average trend (as shown by the dotted line) may be imitated in the balancing network. There is, therefore, always a certain amount of unbalance at some frequencies, even in the most carefully constructed lines. This causes a serious limitation on the amplifying power which may be used, particularly with arepeater which has unequal amplifications at different frequencies, for in this case the amplification must be made so small that at the frequency of the most eflicient operation of the repeater, the conditions implied in the above equation shall not beviolated. The object of the present invention is to so relate the repeater and loaded line characteristics that this difiiculty may be minimized.

The curve of Fig. 3 shows the relation between the frequency f and the amplifying power A of a repeater of the receiver-microphone type.- It will be noted that the point of most efficient amplification lies at a certain definite frequency (71), which is the frequency of resonant response of the vibrating system. Now in any loaded line, such as the line 2 or 3 illustrated in Fig. 1, having lumped inductance L and distributed capacity C, the attenuation offered to cur-- rents in the line is related to the frequency of these currents by a relation which is shown graphically in Fig. 4. It will be noted that the attenuation has a small value for a considerable rangeof frequencies, and that it rises rather abruptly at a certain value of frequency, which may be shown.

theoretically to be equal to l r T6 In this formula, L and (rare respectively 'singing occurs at that amplification.

the inductance and capacity per loading section of the loaded line. y

A comparison of the curves of Figs. 3 and 4, whiclrrepresent respectively 'characteris- Y tics of the repeaterand'of the line, will show that if the resonant frequency f is made to fall below the frequency f;, at which the line begins to offer a high attenuation, the average amplification allowable in the repeater will be small, since the peak of the curve of Fig. 3 must be low enough to insure that no The amplification over the rest of the range will,

therefore, be restricted to a much smaller over, the amplification at various frequencies inside the range of free transmission is made much more uniform because the curve of Fig. 3 is approximately straight, except in the neighborhood of the resonant frequency of the. repeater. In operatingsuch a resonantly responsive repeater on a loaded line, it is therefore essential for high efiiciency that the resonant frequency be, considerably higher than the frequency at which the line begins to offer an increasing attenuation. Where this so-called cutofi frequency is in the neighborhood of 2200 periods per sec- 0nd, as in the case of the average loaded line, a repeater having a moving system, such as that illustrated in Figs. 5 and 6, has been found capable of producing the desired result. In this repeater the moving system consists of the corrugated metal diaphragm 30, which acts as the movable'ele'ctrode for the transmitter element 31, and has at its center a short iron plunger 32 which serves as an armature for the electromagnet of the receiver element 33. The details of this repeater structureneednot be entered. into, as they will be found fully described in Shreeve Patent No. 1,156,636.

It will be noted in connection with Fig. 1. that the repeaters A and B have, in addition to the actuating windings 10 and 1'? respectively, magnetizing windings 26 and 27 and differential windings 28 and 29, whose functions need not be discussed here, as complete description will be found in Shreeve Patent No. 1,159,603 of November 9, 1915.

What is claimed is: I

1. lln a transmission system, the combination with line conductors capable of trans- 15. -pedance of said line for the range of fremitting eficiently electric current within only a limited range of frequencies, of a repeater for reinforcing said currents and having a moving system Whose natural period of vibration lies outside of said range. 2. In a telephone transmission system, the combination with a loaded line, of a telephone repeater having a moving system whose natural period of vibration is greater than the cutoff frequency of said loaded line. 3. In a telephone transmission system, the combination with a telephone line having distributed capacity and lumped inductance, of an artificial line having an impedance characteristic equivalent to the average imquencies which said line transmits efficiently,

. and a telephone repeater having a moving system whose natural period of vibration lies outside said range of frequencies.-

4:. In a telephone transmission system, two telephone lines, eachhaving distributed capacity andlumped inductance, an artificial line for each of said lines and having an impedanceqcharacteristic equivalent to the average impedance of the telephone line individual thereto for the range of frequencies which said line transmits eliectively,

and two telephone repeaters connecting said lines, each of said repeaters'h aving a movingsystem whose natural period of vibration lies outside said effective range of either. of

said lines.

5. In a telephone transmission system, two

loaded lines, an artificial line connected to each of said lines and having an impedance equivalent to that of the loaded line to'which it is connected, and two telephone repeaters connecting said loaded lines, each of said repeaters having a moving system whose natural period of vibration is greater than the cutoff frequency of said loaded lines.

6. In a telephone transmission system,

two loaded lines each terminating in a bal-' ancing artificial line, in combination with two telephone repeaters having input circuits and output circuits, each of said repeaters having its input "circuit connected.

to one of said loaded lines and its output circuit connected to the other of said loaded lines respectively, and having a moving system whose natural period of vibration is greater than the cutoff frequency ofsaid loaded lines,

7. In a telephone transmission system, a repeater station, two telephone lines having lumped inductance and distributed capacity terminating at said station, an artificial line individual to each of said lines and adapted .ers having a moving system whose natural period of vibration lies outside the said effective transmitting range of said telephone lines. a v

In Witness whereof, I hereunto subscribe my name this 23rd day of December, A. D. 1915.

' EDWIN H. COLPITTS. 

